I might be nitpicking, but this:

> A few employers ago, giving a "best guess" rather than admitting you don't know the answer to a question was an instant no-hire.

and this:

> At my most recent one, I was actually encouraged to speculate if I didn't know the answer to a tech question

aren't incongruous to me. In my interviews, I lay out two explicit expectations for the candidate upfront:

1. Please don't bullshit an answer. Confidently incorrect is considered a red flag.

2. The interview is designed to find the ceiling on your certainty and knowledge, and to see how you deal with ambiguity thereafter.

I encourage candidates to guess; I just want them to make it very explicit when they're guessing, and demonstrate self-awareness in doing so.

As ever, a lot of interview problems on both sides of the table are resolved with more communication. It is good to set explicit expectations for candidates throughout the process, and to revisit particular expectations in the first two to three minutes of an interview.